Three main gases are generally present in a carbonated beverage: carbon dioxide (CO2), oxygen (O2) and nitrogen (N2). Carbon dioxide is the desired gas. Oxygen is undesired.
Direct measurement of the amount of a gas in a liquid phase is possible but has some drawbacks. One of them is the necessity for shaking the liquid before the measurement until a state of equilibrium between the liquid phase and the gaseous phase is obtained. Another drawback is that, to perform the measurement, the liquid is to be passed into various sensors and this may affect the reliability of the sensors. This is why another method is proposed in U.S. Pat. Nos. 5,220,513, 5,426,593 and 5,604,297 for measuring the amount of a determined gas, particularly oxygen, in a sealed liquid container, in which method the sealed liquid container is pierced, the pierced container is degassed to release the gases from the liquid phase, using ultrasounds or other degassing means, the released gases are passed into a test chamber, and the amount of the determined gas in the released gases in the test chamber is measured with a specific sensor. However, a problem with that method is that degassing causes foam to be formed at the liquid-gas interface, especially in carbonated beverages such as beer, which foam may wet the probe of the sensor and thus affect the measurement. To remedy this problem, it is proposed in the above-mentioned patents to use a foam chamber between the liquid container and the test chamber to absorb the foam. This solution however has several drawbacks. The use of such a foam chamber increases the risk of contaminating the gas mixture extracted from the liquid with oxygen. Indeed, due to its much larger size than the conduits of the gas circuit, a chamber is more likely to house contaminants that will resist purging. Oxygen that remains in the chamber after the gas circuit has been purged may affect the measurement. Another drawback is that with such a foam chamber the wait time before which the measurement can be made, after piercing the sealed liquid container, may be long because it must be waited until the foam has fallen down in the foam chamber.